Total solar eclipses

A total solar eclipse is one of the most spectacular astronomical phenomena that you will ever witness, this side of a major comet or exploding fireball. The alignment of the Moon in front of the Sun causes the Moon to cast a broad cone-shaped shadow, called the penumbra, onto the Earth. Anyone standing within the penumbra, which is about 6,000 kilometers in diameter, will see a partial eclipse. At the heart of the penumbra lies an even more finely tapered shadow cone, called the umbra. The tip of the umbral shadow cast on to the Earth is usually between 150 and 250 kilometers across near the equatorial regions, and wider as the shadow stretches toward the poles. Anyone standing in the umbra witnesses a total eclipse in which the Sun is completely occulted.

During a total solar eclipse, the Moon's shadow trends west to east across the Earth at about 1,700 kilometers per second; thus the Sun can only be completely eclipsed for a matter of minutes at any one place along the path of totality. Observers first see a small part of the Moon's limb silhouetted against the edge of the Sun's disk at a moment called first contact. The partial phases thus proceed until about an hour to an hour and a half later, when the Moon completely blocks the Sun. This is called second contact, the beginning of totality. The moment just before second contact is often heralded by a dagger-like sunbeam streaming from one spot on the darkened limb. This is known as the diamond ring effect.

As long as the Sun is eclipsed, it is perfectly safe to observe or photograph the darkened disk without solar filters or special protection devices. After all, you're essentially looking at the Moon in silhouette, not the bright surface of the Sun. It is exceedingly dangerous, though, to look at any part of the uneclipsed Sun. A chink of unadulterated sunlight is as bright as any part of the rest of the Sun and can cause permanent eye damage. You should be aware of the duration of the eclipse you're observing, so you can anticipate the Sun's sudden and dazzling reappearance from behind the Moon.

Totality is an uncanny, never-to-be-forgotten experience. The sky grows ever darker until the region immersed in the umbra takes on the hues of twilight. When the Sun is overhead, the sky all around the horizon is tinged with the colors of sunset. Crabs scuttle onto the shore,

A total solar eclipse displaying the diamond ring effect and coronal streamers. (Illustration by Jeff Kanipe.)

birds seek out their nests, roosters crow, and some flowers will even close. If totality occurs along a coastal area, the wind may gust off the water as the temperature suddenly drops. Clouds outside of the path of totality remain brilliantly lit by the partial Sun, but the sky overhead is dark enough to see the planets and the brighter stars.

As totality deepens, the outer atmosphere of the Sun, called the corona, becomes distinctly visible. The corona, comprised of rarefied gases extending millions of kilometers in all directions from the Sun's surface, appears like a milky halo of flower petals. On the limb, you may see bright red tongues of gas arching away from the Sun. These are solar prominences, fountains of energy leaping tens of thousands of kilometers from the Sun's surface.

Third contact occurs some few minutes later (often it seems like only seconds) as sunlight begins streaming through various lunar valleys along the limb, creating another diamond ring or a beautiful string-of-pearls display called Bailey's Beads. The fourth and final contact occurs when the Moon no longer covers the Sun, marking the end of the eclipse.

The fact that we can view such a remarkable event is itself one of Nature's more extraordinary coincidences. Although the Sun is 400 times larger than the Moon, it is also some 400 times further away. As a result, both objects have the same apparent size, half a degree.

Sometimes, however, the Earth is closer to the Sun, as it is in winter months, while the Moon is at or near apogee, its greatest distance from Earth. These circumstances conspire to make the Moon appear slightly smaller than the Sun. When that happens, the Moon can't quite cover the entire solar disk, and the outer limb of the Sun appears as a ring, or annulus, of dazzling light in the sky. This is called an annular eclipse. Although not as dramatic as a total eclipse (and not safe to observe without eye protection) it is still well worth seeing if one occurs in your part of the world. The sky darkens noticeably, and the sight of a ring of sunlight in the sky (viewed through filters or by projecting its image through a telescope) is striking.

Although solar eclipses are predictable events, they don't occur at the same time each year, nor in the same regions of the world. There can be as many as four solar eclipses per year, if you count the partial ones. More often, however, there are two, usually a total and an annular eclipse.

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